[FIX] im_chat : scroll down on opening conversation and add scroll bar on user list
[FIX] im_chat : use direct sql query on im_search method for perfomance issue
 and fix away status
[FIX] crm: avoid constraint errors when merging partners

As for the _update_foreign_keys, the _update_reference_fields method may raise an unique constraint when merging two partners.
In such case, the new record is not relevant and can be removed.
[IMP] website_forum: performance improvements

- avoid to browse on every question/answer, only the 20 most recent ones (need to manually update the view to see the real number of q&a)
- do not render hidden tabs (leakage of information and useless rendering)
- add related fields to speed up vote search (need to be stored to be efficient)
[IMP] models: inherited fields are related fields read as the current user

Add an attribute 'related_sudo' (True by default) for related fields.
A related field is computed as superuser if related_sudo is True.